As a general wafer shape measuring apparatus, these is one having a fundamental principle of holding three edges (provided at intervals of 120°) of a silicon wafer having a diameter of 300 mm by a holding assembly called a strip-like gripper having a thickness of approximately 1 mm and scanning an entire plane of the wafer by an electrostatic capacitance sensor while rotating the wafer, thereby measuring a thickness and a shape of the wafer, for example. A contact part of the gripper which holds the wafer with respect to the wafer has a shape obtained by forming a groove with a curvature radius of 0.7 mm in one linear side of the strip-like shape, and the wafer is fitted and pressed in this groove to prevent the wafer from coming off due to high-speed rotation during measurement (see Japanese Patent Application Laid-open No. 2002-539621).
In the shape measuring apparatus, although the gripper applies a fixed stress to press the wafer in order to present the wafer from coming off due to rotation, the shape of the wafer may be deformed due to the stress at this moment. The grippers are arranged on an outer periphery of the wafer at intervals of 120 degrees so that the stress is uniformly applied to the wafer in order to suppress the deformation to the minimum level.
At this moment, if a central line of the grippers is placed on an extension of a radius of the wafer, the wafer can be gripped at ideal positions, and the deformation of the wafer can be suppressed to the minimum level, thereby excellently maintaining reproducibility of a shape measurement value and correlativity between apparatuses.
However, since an adjustment width is provided to a member called a flexure, which fixes the gripper, the gripper is actually set with a slight inclination, but quantitatively measuring the inclination of the gripper is difficult. When the gripper is inclined, the gripper cannot hold the wafer at the center of the gripper, and holding positions (contact parts between the grippers and the semiconductor wafer) are not provided at intervals of 120 degrees.
Therefore, concentration of the stress occurs in a given direction, and the gripper holds the wafer with an actual shape of the wafer being deformed when the original shape of the wafer interferes with concentration of the stress, thereby deteriorating the reproducibility of a measurement value and the correlativity between apparatuses. Therefore, when the reproducibility of a shape measurement value and the correlativity between apparatuses are not stabilized, there occurs a problem that measurement values become different from each other.
Since controlling an inclination of the flexure as a base portion of the gripper is difficult, maintaining and managing a state where the gripper is not inclined is very difficult.